Pizza needs to be baked thoroughly. Once I undercooked a frozen pizza, and I ended up with a stomachache. Putting the frozen pizza directly on the oven rack, instead of on a cookie sheet, will allow it to be baked more thorough. Since some pizzas do have pork-based toppings (such as sausage, pepperoni, ham, or bacon), they do need to be baked thoroughly. I usually bake it until its brownish on the top, usually 4-5 minutes past the time on the package.